Metal oxide varistor polyphase transient voltage suppression

ABSTRACT

A body of sintered metal oxide material exhibiting highly nonlinear resistance characteristics includes a base and plurality of members projecting therefrom. Electrodes in the form of electrically conductive material are plated on some or all of the major surfaces of the projecting members and base. The electrodes provide connections to electrical conductors connected to the power input or output terminals of a single or polyphase electrical apparatus and the nonlinear resistance characteristics of the metal oxide material provides desired line-to-line and line-to-neutral transient voltage suppression in accordance with the connections of the electrical conductors.

United States Patent Anderson [1 1 3,693,053 51 Sept. 19, 1972 [54] METAL OXIDE VARISTOR POLYPHASE TRANSIENT VOLTAGE SUPPRESSION [72] Inventor: Thomas E. Anderson, Schenectady,

[73] Assignee: General Electric Company [22] Filed: Oct. 29, 1971 [21] App]. No.: 193,963

[52] US. Cl. ..317/23l, 317/238, 317/61 [51] Int. Cl. ..ll01c 7/12, 1102b 3/26 [58] Field of Search ..317/238, 230, 231, 61, 61.5

[56] References Cited UNITED STATES PATENTS 3,320,482 5/1967 Sakshaug et a1 ..3l7/6l 3,358,220 12/1967 Fahlen et al ..3l7/61 3,366,831 l/l968 Lapple ..3 17/61 3,641,394 2/1972 Hirose et al. ..3l7/61.5

Primary Examiner-James D. Kallam Attorney-John F. Ahem et al.

[57] ABSTRACT A body of sintered metal oxide material exhibiting highly nonlinear resistance characteristics includes a base and plurality of members projecting therefrom. Electrodes in the form of electrically conductive material are plated on some or all of the major surfaces of the projecting members and base. The electrodes provide connections to electrical conductors connected to the power input or output terminals of a single or polyphase electrical apparatus and the nonlinear resistance characteristics of the metal oxide material provides desired line-to-line and line-toneutral transient voltage suppression in accordance with the connections of the electrical conductors.

27 Claims, 14 Drawing Figures PATENTED I 9 I973 3.693, 053

SHEET 2 [IF 4 F/ BC /0,000 IIHIII llllllll IIIIHH I IIIIHI I I||H 4000 3 2 u) 3000 /0;z 00000: 10000400@ Q I A Y 00,2; 400

00 l I Illlll l lllIlH l I IIHH I IIH'HI I IIIHII MM /0/24A 0 01,4 /.0A /0A /00A METAL OXIDE VARISTOR POLYPHASE TRANSIENT VOLTAGE SUPPRESSION My invention relates to a polyphase transient voltage suppressor utilizing a varistor device, and in particular, to a varistor device fabricated from a compact body of sintered metal oxide material having highly nonlinear resistance characteristics which provide exceptional voltage limiting characteristics.

Transient voltages resulting from any of a number of causes often occur on polyphase electric power lines and may cause damage to any electrical machine, appliance and the like connected to such power line if the transient is of sufficient magnitude. In particular, transient voltages occurring on the common three phase, four wire power lines can cause damage to three phase and single phase electrical apparatus such as motors and household appliances connected across the three and single phase lines, respectively. Thus, it is readily apparent that excessive peaks of voltage transients must be reduced to levels which assure that the load devices connected across the power lines are not damaged. A recently developed material which exhibits highly nonlinear resistance characteristics, and will be described in greater detail hereinafter, has been found to have exceptional voltage limiting characteristics as well as many other advantages such that it is the basis of a new class of improved transient voltage suppressors. This material, a sintered metal oxide, has a relatively high energy handling capability and is capable of being fabricated into a variety of shapes of various sizes.

Therefore, the principal object of my invention is to provide an improved polyphase transient voltage suppressor.

Another object of my invention is to provide the structure of the suppressor in a simple compact form.

A further object of my invention is to fabricate the suppressor from a single body of metal oxide material.

In accordance with my invention, I provide an improved polyphase transient voltage suppressor which comprises a single body of sintered metal oxide material exhibiting highly nonlinear resistance characteristics. The body includes a base and plurality of members projecting therefrom or a plurality of members only. Electrodes plated on some or all of the major surfaces of the projecting members and base member provide connections to electrical conductors connected at the input or output of an electrical apparatus to be protected against transient voltages. The particular interconnections of the electrical conductors determine the particular line-to-line and, or line-to-neutral protection obtained.v

The features of my invention which I desire to protect herein are pointed out with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation together with further objects and advantages thereof may best be understood by reference to the following description taken in connection with the accompanying drawings wherein like parts in each of the several figures are identified by the same reference character and wherein:

FIG. 1 is a graphical representation of the nonlinear resistance and resultant voltage limiting characteristics of metal oxide and silicon carbide material for different values of the exponent alpha plotted in terms of voltage versus amperes on a log-log scale;

FIG. 2a is a schematic diagram of a circuit application of the metal oxide varistor connected across a load in a circuit supplied by 600 volts with a 1,000 volt transient superimposed thereon;

FIG. 2b is a plot of volts versus amperes on a log-log scale for depicting a graphical comparison of the steady state power dissipation in metal oxide and silicon carbide varistors in the FIG. 2a circuit when clamping the load voltage at 1,200 volts;

FIG. 2c is a plot of volts versus amperes on a log-log scale for depicting a graphical comparison of the voltage clamped across the load for the metal oxide and silicon carbide varistors for a maximum steady state power dissipation of one watt in the varistor;

FIG. 3a is an isometric view of a first embodiment of my polyphase transient voltage suppressor constructed in accordance with my invention;

FIG. 3b is a schematic diagram representation of the structure illustrated in FIG. 3a;

FIG. 4a is an isometric view of a second embodiment of my suppressor;

FIG. 4b is a schematic diagram representation of the structure shown in FIG. 4a;

FIG. 5a is an isometric view of a third embodiment of my suppressor;

FIG. 5b is a schematic diagram representation of the structure shown in FIG. 5a;

FIG. 6a is an isometric view of a fourth embodiment of my suppressor;

FIG. 6b is a schematic diagram representation of the structure shown in FIG. 6a;

FIG. 7 is an isometric view of the structure shown in FIG. 3a with cooling means for higher power applications; and

FIG. 8 is an isometric view of the structure of FIG. 3a with a second embodiment of cooling means.

There are few known materials which exhibit nonlinear resistance characteristics and which resort to the following equation to relate quantitatively current and voltage by the power law:

where V is the voltage between two points separated by a body of the material under consideration, I is the current flowing between the two points, C is a constant and a is an exponent greater than 1. Both C and a are functions of the geometry of the body formed from the material and the composition thereof, and C is primarily a function of the material grain size whereas a is primarily a function of the grain boundary. Materials such as silicon carbide exhibit nonlinear or exponential resistance characteristics and have been utilized in commercial silicon carbide varistors, however, such nonmetallic varistors typically exhibit an alpha (0:) exponent of no more than 6. This relatively low value of alpha represents a nonlinear resistance relationship wherein the resistance varies over only a moderate range. Due to this moderate range of resistance variation, the silicon carbide varistor is often connected in series with a gap when used in a circuit for transient voltage suppression since continuous connection of the varistor could exceed the power dissipation capabilities thereof unless a relatively bulky body of such material is used in which case the steady state power dissipation is a rather severe limitation. An additional drawback is the ineffectiveness of the voltage clamping action as a result of the limited value of silicon carbide alpha exponent. The moderate range of resistance variation results in voltage limitation which may be satisfactory for some applications, but is generally not satisfactory when the transient voltage has a high peak value.

A new family of varistor materials having alphas in excess of within the current density range of 10' to 10 amperes per square centimeter has recently been produced from metal oxides although few applications have been disclosed for this new metal oxide varistor material also referred to herein as MOV, a trademark of the General Electric Co. Although the alpha of the MOV materials, in which range the alpha remains substantially constant, are identified by the current density range of 10' to 10 amperes per square centimeter, it is appreciated that the alphas remain high also at higher and lower currents although some deviation from maximum alpha values may occur. The MOV material is a polycrystalline ceramic material formed of a particular metal oxide with small quantities of one or more other metal oxides being added. As one example, the predominant metal oxide is zinc oxide with small quantities of bismuth oxide being added. Other additives may be aluminum oxide, iron oxide, magnesium oxide, and calcium oxide as other examplesThe predominant metal oxide is sintered with the additive oxide(s) to form a sintered ceramic metal oxide body. Since the MOV is fabricated as a ceramic powder, the MOV material can be pressed into a variety of shapes of various sizes. Being polycrystalline, the characteristics of the MOV are determined by the grain (crystal) size, grain composition, grain boundary composition and grain boundary thickness, all of which can be controlled in the ceramic fabrication process.

The nonlinear resistance relationship of the MOV is such that the resistance is very high (10,000 megohms has been measured) at very low current levels in the microampere range and progresses in a nonlinear manner to an extremely low value (tenths of an ohm) at high current levels. The resistance is also more nonlinear with increasing values of alpha. These nonlinear resistance characteristics result in voltage versus current characteristics wherein the voltage is effectively limited, the voltage limiting or clamping action being more enhanced at the higher values of the alpha exponent as shown in FIG. 1. Thus, the voltage versus current characteristics of the MOV is similar to that of the Zener diode with the added characteristic of being bidirectional and over more decades of current. The conduction mechanism of the MOV is not yet clearly understood but it is completely unlike the avalanche mechanism associated with the Zener diodes, a possible theoretical explanation of its operation being that of space charged limited current. The rated voltage and the voltage range over which the varistor effect occurs are determined by the particular composition of the MOV material and the thickness to which it is pressed in the fabrication process. The MOV includes conduction changes at grain boundaries resulting in the advantage of bulk phenomenon allowing great flexibility in the design for specific applications simply by changing the dimensions of the body of MOV material. That is, the current conduction in the absence of closely spaced electrodes along one surface of the MOV body is through the bulk thereof. The bulk property of the MOV also permits a much higher energy handling capability as compared to junction devices. Thus, since an MOV device can be built to any desired thickness, it is operable at much higher voltages than the Zener diode junction device and can be used in a range from a few volts to several kilovolts. The voltage changes across a silicon carbide varistor device are much greater than across an MOV device for a given current change as seen in FIG. 1 for the plot designated a 4 and thus the silicon carbide varistor has a much smaller voltage operating range thereby limiting its applications as described hereinabove. The thermal conductivity of MOV material is fairly high (approximately one-half that of alumina) whereby it has a much higher power handling capability than silicon carbide, and it exhibits a negligible switching time in that its response time is in the subnanosecond domain. Finally, the MOV material and devices made thereof can be accurately machined, soldered, and operated at very low voltages, capabilities not possible for the larger grained silicon carbide.

The voltage versus current characteristics plotted in FIG. 1 of the drawings illustrate the nonlinear or exponentjal resistance characteristics exhibited by varistor material, and in particular, the increasing nonlinearity and enhanced voltage limiting obtained with increased values of the exponent alpha (a) wherein the top line a 4 is typical for silicon carbide varistors and the three lines a 10, 25 and 40 apply to varistors fabricated of MOV material. The VOLTS abscissa is in terms of the voltage appearing across the terminals of a specific MOV device in response to current flowing through the bulk of the MOV material and represented along the CURRENT ordinate. Although the use of linear scales on the graph would show the decreasing slopes (decreasing resistance values) with increasing currents, such curves can be readily manipulated by the choice of scales, and for this reason, log-log scales are chosen to obtain a family of lines each of which remains substantially straight within the indicated current range. It can be seen from the FIG. 1 plots that the resistance exhibited by the MOV material is quite high at low current levels and becomes increasingly smaller in a nonlinear exponential manner with increasingcurrent levels. Extension of the plots to lower and higher current levels would obviously indicate correspondingly much higher and lower resistances, respectively, and operation of the MOV device may transiently reach such levels depending upon the particular circuit application of the device. For purposes of comparison, each of the volts versus amperes plots passes through the point identified by volts and l milliampere. It should be understood that metal oxide materials (zinc oxides) are available having alpha exponents even greater than 40 which thereby obtains even greater enhanced voltage clamping action than that exhibited for the a 40 line.

Referring now to FIG. 2a, there is shown a simple direct current circuit utilizing a varistor connected across a load and serving the basis for the graphs of FIGS. 2b and 2c to be described hereinafter. In particular, the circuit includes a load 20 connected in series with a 600 volt d.c. source and a source 21 producing a 1,000 volt transient. The voltage transient is assumed to be a square wave pulse of millisecond duration. The internal impedance of the 600 volt source is represented by series resistor 22 of 10 ohms. A varistor device 23 is connected across load for clamping or limiting the voltage thereacross to a desired value. The direct current is used for convenience to simplify the example, and is also valued for alternate current operation when properly analyzed.

Referring now to FIG. 2b, there are drawn the resistance lines which illustrate the operating characteristics of the circuit illustrated in FIG. 2a. In particular, the upper 10 ohm source impedance line indicates the voltage developed across load 20 with increasing current flow therethrough, the maximum 1,600 volts being the summation of the steady state 600 and transient 1,000 volts. For a current flow of 40 amperes, the load voltage is 1,200 volts due to the 400 volt drop across the 10 ohm resistor 22. In the case wherein a silicon carbide varistor 23 is connected across load 20 for clamping the voltage at 1,200 volts for a current flow of 40 amperes, the load line for such varistor having an alpha exponent equal to 6 crosses the steady state 600 volt line at 0.6 ampere resulting in a steady state power dissipation of 360 watts in the varistor device. In comparison, a metal oxide varistor of the same clamping voltage-at 40 amperes fabricated of zinc oxide material having .an alpha exponent of has a steady state power dissipation of a mere 90 milliwatts. Thus, the metal oxide varistor employed in my invention has a steady state power dissipation which is in the order of one four-thousandths of the dissipation in a silicon carbide varistor. Obviously, a metal oxide varistor having an alpha exponent in excess of 25 would have a steady state power dissipation even less than the 90 milliwatts. It can be appreciated that the 360 watt steady state power dissipation in a silicon carbide varistor is virtually unbearable and would quickly result in destruction of the varistor device unless such device was of such massive volume that it would have the requisite energy handling capability, a completely unpractical consideration. Also, it should be noted that the more typical, commercially available silicon carbide varistors have alpha exponents of 3 to 4 whereby the problem associated therewith is even worse.

Another convenient manner in comparing the characteristics of the metal oxide and silicon carbide varistor devices is illustrated in FIG. 20 wherein a maximum allowable steady state power dissipation of one watt is maintained for both varistor devices. In the case of the silicon carbide varistor, (a 6) the volt-ampere characteristic line intersects the 10 ohm source impedance line at the voltage level of 1,594 volts, that is, the silicon carbide varistor is capable of clamping the load voltage at only 1,594 volts for a maximum applied voltage of 1,600. In comparison, the metal oxide varistor having an alpha exponent of 25 and identical 1 watt steady state power dissipation is capable of clamping the voltage across the load at 940 volts, an improvement of 654 volts over the silicon carbide varistor. Thus, the silicon carbide varistor has suppressed the applied 1,600 volts by a mere 6 volts whereas the metal oxide varistor has suppressed it by 660 volts. The FIGS. 2b and 2c graphs clearly indicate the superior steady state power dissipation and voltage clamping characteristics obtained by the metal oxide material as compared to a silicon carbide varistor. These superior characteristics are utilized in the polyphase transient voltage suppressor to be described hereinafter in accordance with my invention.

Referring now to FIG. 3a, there is shown a first embodiment of my polyphase transient voltage suppressor which provides three phase line-to-line and line-toneutral protection as indicated in the schematic diagram representation of the device in FIG. 3b. My suppressor consists of a body of sintered metal oxide material exhibiting highly nonlinear resistance characteristics and the structure includes a base member 30 and three members 31, 32 and 33 projecting therefrom.

As one example, the sintered metal oxide material may consist primarily of zinc oxide and a small percentage of bismuth oxide. Base 30 and projecting members 31, 32 and 33 are preferably formed as a single body by being molded in the desired shape. Alternatively, the projecting members and base may be fabricated as separate elements and then joined together to form the unitary structure depicted in FIG. 3a although this latter approach would appear to introduce many difficulties, would probably be more costly and could have operating characteristics inferior to that of the single formed body. Thus, it is to be understood that the body of MOV material fabricated from separate elements which are thence joined to form a unitary structure is also considered to be in the scope of my invention although it is not the preferred embodiment. The number of projecting members, illustrated as three in FIG. 3a, is determined by the number of phases to be protected against transient voltages. Thus, six projecting members would be utilized in the case of a six phase transient voltage suppressor.

For purposes of structural rigidity and simplicity of fabrication, the projecting members 31, 32 and 33 have a common juncture coaxial with the centerline axis of base 30 and project radially outward from the juncture. Base 30 and projecting members 31, 32 and 33 each include a pair of opposed major surfaces which are generally flat and parallel to each other. Thus, base 30 includes first major surface 30a and a second major surface parallel thereto and forming the back (unseen) end of the suppressor device depicted in FIG. 3a. In like manner, projecting member 31 includes first major surface 31a and a second major surface parallel thereto but unseen in FIG. 3a. Projecting member 32 includes visible first major surface 320 and an unseen second major surface parallel thereto, and a projecting member 33 includes first major surface 33a and an unseen second major surface parallel thereto. Base 30 may have any of a number of forms and is depicted in the FIG. 3a embodiment as being circular in cross section. Examples of other forms of the base member are the triangular form illustrated in FIG. 5a and the square or rectangular form in FIG. 6a. Projecting members 31, 32 and 33 may also be any of a number of shapes, but for convenience of fabrication by the molding process and for purposes of forming a compact body, these members are of rectangular or square shape. For these reasons also, the side surfaces (31b and 33b are visible) of the projecting members 31, 32 33 and the side surface 30b of base member 30 are continuous, that is, are flush with each other, the side surfaces of the projecting members being curved in conformance with the circular curved surface of the particular base member illustrated in FIG. 3a. And again for the above reasons, the end surfaces 31c, 32c, 330 of projecting members 31, 32, 33 are coplanar and parallel to the major surfaces of the base 30. The projecting members may be arranged in any of a number of orientations on the base member, but for ease of fabrication, they are arranged in a Y-shaped pattern (120 orientation) in FIGS. 3a, 4a, 5a and in T-shaped pattern in FIG. 64 as typical examples for a three phase suppressor.

Electrodes are provided on the major surfaces of the base and projecting members for providing connections to electrical conductors that are connected to a polyphase power line or to the input or output electric power terminals of an electrical apparatus being protected from voltage surges by my transient voltage suppressor. The electrodes are in the form of metallized surfaces which are plated on the major surfaces of the base and projecting members for providing good electrical and mechanical contact therewith. The metallized surfaces are obtained by a suitable bonding process which may be accomplished by thick film techniques or by pressure contacts, as two examples. The metallized surface may be obtained by firing a thin layer of silver-glass frit (silver and glass particles) on the MOV major surfaces. Ohmic contact is utilized in order to take advantage of the bulk phenomenon operation of the MOV material. As depicted in FIG. 3a, electrodes are plated on each of the major surfaces of the metal oxide varistor body. In particular, metallized surface or electrode 34 is formed on the first major surface 31a of projecting member 31 and a second such metallized surface of identical form isformed on the opposite major surface (not seen) of member 31. In like manner, the two major surfaces of projecting member 32 are provided with two metallized surfaces, one of which 35 is visible in FIG. 3a and the two major surfaces of projecting member 33 are provided with two metallized surfaces, one of which 36 is visible. Finally, base member 30 is provided with metallized surfaces along each of the three 120 sectors on major surface 30a, two of which 37 and 38 are visible in FIG. 3a. The other (unseen) major surface of base member 30 which defines the back end of the device is provided with a metallized surface along substantially all of its area except along the edges. For purposes of assuring that current conduction will be through the bulk of the MOV body (bulk operation), the edges of the metallized surface electrodes terminate at some predetermined distance from the side and end surfaces of the projecting members 31, 32 and 33 and base member 30. In the case of the FIG. 3a embodiment, the transient voltage suppressor device provides three phase line-to-line and line-to-neutral protection and for this reason the group of three metallized surfaces in each of the 120 portions of the device are interconnected as shown, that is, the three adjoining electrodes are formed as a single continuous metallized surface electrode. Thus, metallized surfaces 34, 36 and 37 are formed as a first single metallized surface electrode and thus are all at the same potential during operation of the device. Alternatively, electrodes 34, 36 and 37 could be separate but interconnected by means of electrically conducting leads. In like manner metallized surfaces 35, 38 and the unseen metallized surface on the unseen major surface of member 33 are formed as a second single metallized surface electrode. Suitable electrical conductors (leads) 39,40, 41 have first ends connected to the electrodes in corresponding portions of the device and a fourth lead 42 is connected to the electrode on the opposite (unseen) major surface of base member 30 to provide the three phase, four wire input to the device. The leads may be connected to the electrode surface along any point thereof, a convenient connecting point for the three phase input being on the 120 sectors of base member 30 as illustrated in FIG. 3a. The remote ends of leads 39, 40 and 41 would be connected to a three phase power line or to the three phase power input or output terminals of the apparatus being protected against transient FIGS.

and the remote end of lead 42 connected to the neutral which may be grounded.

thicknesses T T and T of projecting members 31, 32 and 33, respectively, as:

1 1 1 iif;v fivi T3 Obviously, the device depicted in FIG. 3a can also b used for merely three phase line-to-line protection by disconnecting lead 42. In this latter application, the entire base member 30 could be omitted and the device consist only of the projecting members. Finally, the device may also be used for single and two phase applications by utilizing the proper leads. Thus, for a single phase application, two leads associated with opposite major surfaces of one of projecting members 31, (leads 39, 41), 32, (leads 40, 41), 33 (leads 39, 40) or base 30 (leads 42 and one of 39, 40, 41) are connected to the single phase input or output terminals of the single phase apparatus being protected. The choice of utilizing the MOV body of a projecting member or the thinner base member in a single phase application is determined by the voltage rating required.

In the case of a two phase transient voltage suppressor device, two leads associated with opposite major surfaces of one of projecting members 31 (leads 39, 41), 32 (leads 40, 41), or 33 (leads 39, 40) and base 30 (lead 42) are connected to form the input or output terminals of a two-phase voltage transient suppressor.

The thickness dimensions of the projecting members and base are related as:

It noted thatwith proper design, considering apparatus tolerances, a two phase suppressor can be made from a three phase suppressor as shown in FIG. 3a but with optimum thickness T Also, in an n polyphase system, the optimumthickness dimensions are related as:

T T T,,=2 T sin 1r/n wheren is the number of phases.

The dimensions for a typical three phase, four wire transient voltage suppressor constructed in accordance with my invention as illustrated in FIG. 3a and adapted for voltage clamping at 240 volts RMS line-to-line is as follows: assuming the metal oxide material has an alpha exponent of 25, the voltage rating of the material is 80 volts RMS per millimeter. Thus, the thickness of each of projecting members 31, 32 and 33 is 240/80 3 millimeters for line-to-line protection and the thickness of base member 30 is 3/ VT millimeters for line-toneutral protection. The metallized surfaces (electrodes) are each of approximately 0.001 inch thickness. The diameter of the base member 30 and length of projecting members 31, 32 and 33 is determined primarily by the maximum power dissipation anticipated. Thus, in the case of anticipated long duration, high peak value transients, or highly repetitive transients, the diameter and length dimensions are made larger than for short duration and, or lower peak value or less frequent transients. As a typical example, for the above described 240 volt suppressor, the diameter and length dimensions may each be in the order of 1 inch.

FIG. 3b is a schematic representation of the three phase, four wire suppressor device illustrated in FIG. 3a. In particular, each metal oxide varistor portion of the device is illustrated schematically as a separate varistor for indicating each phase-to-phase and phaseto-neutral circuit. Thus, the particular varistor formed by projecting member 31 and the pair of metallized surface electrodes plated on opposite major surfaces thereof is depicted in the schematic diagram as varistor 31. In like manner, projecting members 32 and 33 and their metallized surfaces are indicated as separate varistors 32 and 33 connected in a delta (A) configuration with varistor 31 for providing phase-to-phase lineto-line protection. Finally, the varistors formed by the three 120 sector portions of base member 30 and their associated metallized surface electrodes are depicted as three separate varistors 30 connected in a Y-configuration with a grounded (neutral) center point for providing line-to-neutral protection, with all members forming the full line-to-line, line-toneutral varistors.

Referring now to FIG. 4a there is shown a second embodiment of my metal oxide varistor polyphase transient voltage suppressor which is a structure similar to that illustrated in FIG. 3a with the exception that each group of three adjoining metallized surfaces (electrodes) are separated rather than forming a single continuous surface as in FIG. 3a. In fact, the metallized surfaces along the major surfaces of the projecting members 31, 32. and 33 may be omitted since they serve no useful purpose in the FIG. 4a embodiment which provides three phase line-to-neutral protection, Y-connection, but does not provide the additional lineto-line protection, delta (A) connection, of the FIG. 3a embodiment. Thus, in its most simple form, the three phase line-to-neutral transient voltage suppressor would merely comprise a base member 30, a metallized surface along a first (the unseen) major surface thereof and having lead 42 connected thereto, and three separated 120 sector shaped metallized surfaces 37, 38 (and one unseen) along the second major surface 30a and having leads 39, 40 and 41 respectively connected thereto. The projecting members 31, 32, 33 are illustrated in FIG. 4a for purposes of utilizing a body of metal oxide material which is identical to that illustrated in FIG. 3a, that is, for standardization purposes, it being recognized that the simpler structure would be less costly if the fabrication process easily permitted construction of the two different type structures.

It should be evident that the FIG. 4a device can be utilized for transient voltage protection for a delta" (A) connected configuration utilizing only the three projecting members 31, 32, 33, and of one or more than one single phase apparatus, actually up to six apparatus by providing leads on each of the electrodes and thereby utilizing the three projecting members 31, 32, 33 and three sector portions of base member 30 as six separate bodies of MOV materials which are operatively substantially independent of each other.

Although. the thickness dimensions of projecting members 31, 32 and 33 are generally equal, this is not always required, and may in some cases not be desired. Thus, in the case wherein the device is utilized for the protection of two to six single phase electrical apparatus operating at two to four different voltage levels or operating at the same voltage level but requiring two to four different voltage protection levels, the thicknesses of the projecting members would be different and determined by the voltage ratings required.

FIG. 4b is a schematic representation of the three phase line-to-neutral structure illustrated in FIG. 4a wherein varistor devices 30 again comprise the three 120 sector portions of base member 30 and their associated metallized surfaces.

Referring now to FIG. 5a, there is shown a third embodiment of my MOV polyphase transient voltage suppressor, and in particular, an embodiment wherein lineto-ground protection is desired for all three phases but only one line-to-line protection is required, or desired, that is, suppression is obtained only between selected lines. The FIG. 5a embodiment also illustrates the use of a different shaped base member 30, such base member being of generally triangular shape. The projecting members 31, 32, 33 extend outward to the corners of the triangle formed by the base. The metallized surfaces 37, 38 (and unseen third metallized surface) on the seen first major surface 30a of base member 30 may conveniently be formed in triangular shapes as illustrated, in 120 sector shapes as in FIG. 4a, or in any other desired form. The group of three metallized surfaces 34, 36, 37 in the first 120 sector portion of the device in FIG. 5a are spaced apart similarly to the same metallized surfaces in the FIG. 4a embodiment. Each of the two remaining groups of three metallized surfaces in the other two 120 sector portions are formed as single continuous metallized surfaces as in the FIG. 3a embodiment, or as separate surfaces as in the first 120 sector portion with leads 40 and 41 connected to their associated three separate metallized surfaces. The three phase leads 39, 40, and 41 may be connected to the three 120 sector metallized surfaces on base member 30 as in the FIGS 3a and 4a embodiments although leads 40 and 41 could be connected to the adjoining metallized surfaces on the adjacent projecting members.

FIG. b is a schematic representation of the structure illustrated in FIG. 5a wherein metal oxide varistor 32 provides the one line-to-line protection.

All of the hereinabove described embodiments of my MOV polyphase transient voltage suppressor have illustrated the three projecting members oriented to form a Y-shape and thereby provide 120 sectors on the visible major surface 30a of the base member. Such orientation of the projecting members is not a requirement, and the fourth embodiment of my device in FIG. 6a illustrates another orientation of such projecting members, this time forming a T-shape. Also, the base member 30 is illustrated as being of square or rectangular shape, it being appreciated that the three areas of metallized surfaces on the visible major surface of base member 30 are preferably equal. The third portion 300 of the visible major surface of base member 30, although not visible in the FIGS. 3a, 4a, 5a embodiments, is plated with metallized surface 60 on which lead 41 is connected. Metallized surface 60 contacts metallized surface 62 on major surface 32a of projecting member 32 and a metallized surface on the unseen major surface of projecting member 31 which forms one-half of the top of the T. In like manner, the three (unseen) metallized surfaces to which lead 40 is connected form a continuous metallized surface. However, the continuous coplanar metallized surface portions 34 and 36 are separated from adjacent metallized surface 37 on base member 30a.

FIG. 6b is a schematic diagram of the structure illustrated in FIG. 6a and indicates that, with the exception of diodes 61, this schematic diagram is the same as that depicted in FIG. 3b. The insertion of diodes 61 in the two lower illustrated line-to-line legs of the delta-connected varistors causes the two line-to-line circuits common with lead 39 to be positive polarity sensitive, that is, transient voltage suppression of both polarity transients occurs on the line 40-to-line 41 and all three line-tomeutral circuits, but negative polarity voltage transients are tolerated on the line 39-line 40 and the line 39-line 41 circuits. The polarity-sensitive phase protection is achieved on the structure illustrated in FIG. 6a by mounting a single diode device 61 on the metallized surface 34 plated on projecting member 31 or on the coplanar metallized surface 36 on projecting member 33 wherein members 31 and 33 form the top of the T formed by the three projecting members. Alternatively, diode 61 could be mounted on metallized surface 37 on the major surface portion 30a of base member 30. The diode is preferably mounted on a metallized surface rather than on a side surface of the device in order to provide protection to the diode from accidental jarring of the side surface against another object. The cathode electrode of diode 61 is connected to the metallized surface portion 34 or 36, and the anode electrode is connected to the metallized surface 37. Since the metallized surfaces 34 and 36 are common on the common major surface of projecting members 31 and 33, the single diode 61 appears operationally (and schematically) in both of the line-to-line connections which are common with the line associated with lead 39. Obviously, the sensitive phase may be made sensitive to the opposite polarity voltage transients by reversing the interconnection of the electrodes of diode 61 in the structure of FIG. 6a.

Although FIG. 6a illustrates the use of only one semiconductor device, it should be obvious that more than one diode, or one or more other type semiconductor device may be mounted on my device to obtain desired circuit functions.

The metal oxide bodies illustrated in FIGS. 3a-6a may be fabricated by any number of methods, one suitable method being the use of a double-acting press for simultaneously forming the base and projecting members into a unitary structure.

Referring now to FIG. 7, there is shown the structure of FIG. 3a with the addition of a plurality of 120 sector cooling fins connected in parallel relationship to the major surfaces of base member 30 and a second set of cooling fins 71 associated with the far (unseen) major surface of base member 30. The cooling fins 70 and 71 may be fabricated of a suitable electrically conductive, high thermally conductive metal such as copper or aluminum. The electrical conductivity of the cooling fins 70 and 71 permits the use of the outermost fins as electrodes for connection to the leads 39, 40, 41 and 42 which have their remote ends connected to a power line or apparatus being protected. The cooling fins 70- may be suitably attached to or through the metallized surfaces on the projecting members and the thinness of the metallized surface permits good heat transfer from the body of metal oxide material to the cooling fins. In the case of the base member cooling fins 71, a core 72 which may be of the same material as fins 70, 71 acts as the heat transfer agent from base member 30 to fins 71. This core 72 may also be utilized along fins 70 if desired, for improving the heat transfer. In either case, core 72 has its side surfaces in contact with side surfaces of adjacent cooling fins and its base in contact with the metallized surface between adjacent cooling fins. The cooling fins would be utilized in large steady state power dissipation applications, that is, when the clamping voltage is very close to the steady state operating voltage, resulting in large steady state power dissipation in the MOV material, and in the case of anticipated long duration and, or high peak value voltage transients or the case wherein transients are repetitive in rather quick succession. My suppressor without the use of cooling fins is generally adequate to cope with the normally isolated transient voltages that occur on the three phase power line. The radius of each cooling fin may be less than, equal to, or greater than the radius of the base member 30, as desired or required by the particular application. Forced air can be provided over the cooling fins to increase the heat transfer,- if necessa- FIG. 8 illustrates a second embodiment of cooling fins for the structure illustrated in FIG. 3a. In the FIG. 8 embodiment, the cooling fins 70 are oriented radially with the juncture of projecting members 31, 32 and 33 and may each conveniently be in rectangular form as illustrated. Each group of fins and the 120 core portion from which they radiate may be a single body fabricated by extrusion. Obviously, the outer edge of each cooling fin in the FIG. 7 embodiment and the three outer edges of each fin in the FIG. 8 embodiment may have shapes other than that illustrated. In FIG. 8,

as well as in FIG. 7, the leads 39, 40 and 41 connecting the device to a three phase power line or the three phase terminals of an apparatus being protected against transient voltages may be connected to the metallized surfaces on projecting members 31, 32 and 33 or to like surfaces on the 120 sector portions on base member 30, although they may also be connected to the cooling fins which are in electrical and mechanical contact with the metallized surfaces. The lead connection on the cooling fins may be preferred in some cases, especially high power applications, wherein the cooling fins would .generally be at a lower operating temperature than the body of metal oxide material and the metallized surfaces plated thereon. The radial cooling fins in the FIG. 8 embodiment, as in the case of the FIG. 7 embodiment, may also extend beyond, be equal to, or less than the radius of base member 30. Cooling fins may also be associated with base member 70 as in the case of FIG. 7, and may be of the form illustrated therein, or radial as illustrated for fins 70 in FIG. 8.

The thickness dimensions of the base and projecting members of the body of metal oxide material are generally selected in accordance with the above equation for a voltage rating at a desired level above the circuit rated voltage at which the voltage clamping or suppression action will occur. Thus, the thickness of such members may be selected for a voltage rating (line-toline for projecting members 31, 32 and 33 and voltageto-neutral for the base member) which is in the order of percent above the circuit rated voltage, as one example. The principal'advantage of my invention is, of

course, the superior transient voltage suppression obtained with the use of a metal oxide varistor. This superior voltage suppression is obtained primarily due to the following three exceptional properties of MOV material: (1) the resistance characteristics are highly nonlinear (alpha greater than 10) over a wide range of current and result in a high degree of voltage limiting, (2) the response time is negligible, and (3) the relatively high thermal conductivity permits rapid dissipation of heat developed in the MOV material due to the voltage transient. As can be seen in the FIG. 1 graph, during the transient (high) voltage condition, the MOV material provides a relatively low resistance path for the current which thence decays at a rate determined primarily by the L/R or RC time constant of the circuit, the resistance of the MOV increasing substantially as the voltage, and current, are decreasing. During steady state circuit operation, the MOV exhibits a relatively high resistance and low power dissipation and has a negligible effect on the circuit operation. The very compact nature of my transient voltage suppressor permits it to be mounted at the input or output power terminals of the apparatus being protected in any convenient manner which may even include merely having the device hang from the power terminals of the apparatus by means of the connecting leads 39, 40, 41 and 42 in the case wherein the apparatus is not subjected to severe mechanical vibration. In the case of such vibration, or for other reasons, a suitable mounting means can be connected on base member 30 or one of the projecting members for providing a rigid support for the device on or adjacent to the apparatus being protected. Another major advantage of my invention is that since all the devices can have the same shape for three phase systems, single type devices, but of various sizes (especially in the thickness of the MOV) can easily be molded to accommodate various voltage ranges.

Having described several embodiments of a metal oxide varistor polyphase transient voltage suppressor, it should be obvious that the base member (if employed) and projecting members of the body of MOV material can assume any of a number of shapes and the metallized surfaces can be coordinated to obtain desired line-to-line and line-to-neutral transient voltage suppressions. The high resistance of the MOV body during circuit steady state operation permits only a small current flow therethrough and therefore results in very low steady state power dissipation as indicated in FIGS. 2b and 2c. In view of this low steady state power dissipation, the MOV body is physically small for the energy handled and thereby permits the connection or mounting of the device very close to the apparatus being protected from the transient voltages. The use of the metal oxide varistor for transient voltage suppression provides a device having many advantages over one wherein other components such as selinium barrier layer rectifiers, Zener diode junction devices or silicon carbide devices are employed since each of such components have at least one of the following limitations which is not present in the metal oxide varistor: the nonlinear resistance relationship varies only a moderate range, the power dissipation in the steady state operation of the circuit is excessive, the voltage clamping action is not adequate, the component does not have bulk properties and therefore is not applicable for power circuit applications, the component has polarity restrictions which are not usually favorable for mounting in conjunction with a semiconductor device, the component is not available for high voltage applications. Thus, while my invention has been particularly shown and described with reference to specific embodiments thereof, it should be obvious by those skilled in the art that obvious changes in form and detail may be made without departing from the scope of the invention as defined by the following'claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A polyphase transient voltage suppressor comprismg a body of sintered metal oxide material including a plurality of members for protecting a plurality of polyphase conductors against transient voltages, each said member provided with a pair of parallel major surfaces, and

electrode means connected to said body along each of a plurality of said major surfaces thereof, said electrode means being connectable to one of the polyphase conductors of an electrical apparatus to be protected against polyphase voltages, said sintered metal oxide material having nonlinear resistance characteristics for limiting the voltage appearing across selected portions of said body defined by said electrode means and for providing a relatively low resistance path for current therethrough during a transient voltage state on the conductors and for providing a relatively high resistance path during steady state operation thereof, the sintered metal oxide material body having energy absorbing properties whereby said suppressor controls transient voltages on the conductors.

2. The polyphase transient voltage suppressor set forth in claim 1 wherein said body of sintered metal oxide material further includes a base member, said plurality of members projecting from a major surface of said base member.

3. The polyphase transient voltage suppressor set forth in claim 2 wherein said base member and said plurality of projecting members are formed from a single body of the sintered metal oxide material to form a unitary structure.

4. The polyphase transient voltage suppressor set forth in claim 2 wherein said base member and said plurality of projecting members are each separately formed from sintered metal oxide material, the members being connected to form a unitary structure.

5. The polyphase transient voltage suppressor set forth in claim 2 wherein said base member has a pair of parallel major surfaces, said plurality of members projecting from a first of the major surfaces of said base member.

6. The polyphase transient voltage suppressor set forth in claim 2 wherein forth in claim 13 wherein the end surfaces of said projecting members being coplanar to form a continuous flatsurface parallel to the major surface of said base member. 15. The polyphase transient voltage suppressor set forth in claim 14 wherein said plurality of projecting members each being of rectangular shape. l6. The polyphase transient voltage suppressor set forth in clam 5 wherein said plurality of projecting members are three in number for fomiing a three phase transient voltage v suppressor. 17. The polyphase transient voltage suppressor set forth in claim 16 wherein said three projecting members being oriented at 120 angles around the center of the major surface of said base member whereby the end surfaces of said three projecting members form a Y-shape. 18. The polyphase transient voltage suppressor set forth in claim 1 wherein the sintered metal oxide material has an alpha exponent in excess of 10. 19. The polyphase transient voltage suppressor set said plurality of projecting members are each of forthin claim 1 wherein equal thickness dimension. 7. The polyphase transient voltage suppressor set forth in claim 1 wherein said plurality of projecting members are of unequal thickness dimension. 8. The polyphase transient voltage suppressor set forth in claim 6 wherein base member is of thickness dimension equal to H2 sin(1r/nsaid projecting members where n is the number of phases. 9. The polyphase transient voltage suppressor set forth in claim 2 wherein said plurality of members project radially outward along the major surface of said base member. 10. The polyphase transient voltage suppressor set forth in claim 9 wherein said plurality of projecting members have a common junction projecting perpendicularly outward from the center of the major surface of said base member. 11. The polyphase transient voltage suppressor set forth in claim 10 wherein said base member is a right circular cylinder having a pair of parallel major surfaces, and the plurality of projecting members each having a curved side surface which are continuous with the side surface of said base member and perpendicular to the major surface of the base member. 12. The polyphase transient voltage suppressor set forth in claim 2 wherein said plurality of projecting members each having a side surface projecting from a side surface of said base member in a direction perpendicular thereto.

13. The polyphase transient voltage suppressor set forth in claim 10 wherein said plurality of projecting members each have an end surface remote from said base member, the end surfaces of said plurality of projecting members each being parallel to the major surface of said base member.

14. The polyphase transient voltage suppressor set the sintered metal oxide material is comprised primarily of zinc oxide. 20. The polyphase transient voltage suppressor set forth in claim 1 wherein said electrode means comprise an electrically conductive material formed on the major surfaces of said plurality of projecting members. 21. The polyphase transient voltage suppressor set forth in claim wherein the electrically conductive material on adjoining major surfaces of adjacent projecting members are interconnected to form a like plurality of electrodes, and a like plurality of electrical conductors having first ends connected to the electrodes and second ends connected to the polyphase power lines to thereby provide polyphase line-to-line transient voltage suppression. 22. The polyphase transient voltage suppressor set forth in claim 5 wherein said electrode means comprise an electrically conductive material formed on the major surfaces of said plurality of projecting members and on portions of said first major surface of said base member, and on a second of the pair of major surfaces of said base member, the electrically conductive material on adjoining major surfaces of adjacent projecting members and on adjoining portions of the first major surface of said base member being interconnected to form a like plurality of electrodes, and like plurality of electrical conductors having first ends connected to the electrodes and second ends connected to the phase lines of the polyphase power line, and additional electrical conductor having a first end connected to the electrode on the second major surface of the base member and a second end connected to the power line neutral to thereby provide polyphase line-to-line and lineto-neutral transient voltage suppression. 23. The polyphase transient voltage suppressor set 1 forth in claim 21 wherein said plurality of projecting members are each of equal thickness dimension and are three in number for forming a three phase transient voltage suppressor,

the electrically conductive material on adjoining major surfaces of adjacent projecting members being interconnected to form three electrodes, and

three electrical conductor having first ends connected to the electrodes and second ends connected to a three phase power line or three phase power input or output of an apparatus to be protected to thereby provide three phase line-to-line transient voltage suppression.

24. The polyphase transient voltage suppressor set forth in claim 22 wherein said plurality of projecting members are each of equal thickness dimension and are three in number for for phase three phase transient voltage suppressor,

the electrically conductive material on adjoining portions of the first major surface of said base member being interconnected with the electrically conductive material on adjoining major surfaces of adjacent projecting members, and

said plurality of electrical conductors are three in number, the additional conductor being a fourth conductor having a first end connected to the electrode formed on the second major surface of said base member and a second end connected to the three phase power line neutral to thereby provide three phase line-to-line and line-to-neutral transient voltage suppression.

25. The polyphase transient voltage suppressor set L forth in claim 24 wherein the electrically conductive material on adjoining major surfaces of at least one pair of adjacent projecting member are not being interconnected and also separate from the electrically conductive material on the adjoining portion of the first major surface of said base member, the electrical conductor associated therewith being connected to the electrically conductive material on the adjoining portion of the first major surface of said base member, whereby selected line-to-line transient voltage suppression is obtained. 26. The polyphase transient voltage suppressor set forth in claim 25 and further comprising at least one semiconductor device mounted on said suppressor and having a first power electrode connected to the electrically conductive material on at least one of said one pair of adjacent projecting members and having a second power electrode connected to the electrically conductive material on the separated adjacent portion of the first major surface of said base member whereby the phase of the power line associated therewith is polarity sensitive to voltage transients on the two other phase conductors. 27. The polyphase transient voltage suppressor set forth in claim 5 wherein said electrode means comprise an electrically conductive material formed on the major surfaces of said plurality of projecting members and base member, and further comcoolin g i'iie ans associated with said plurality of projecting members and base member for obtaining higher power operation of the suppressor. 

2. The polyphase transient voltage suppressor set forth in claim 1 wherein said body of sintered metal oxide material further includes a base member, said plurality of members projecting from a major surface of said base member.
 3. The polyphase transient voltage suppressor set forth in claim 2 wherein said base member and said plurality of projecting members are formed from a single body of the sintered metal oxide material to form a unitary structure.
 4. The polyphase transient voltage suppressor set forth in claim 2 wherein said base member and said plurality of projecting members are each separately formed from sintered metal oxide material, the members being connected to form a unitary structure.
 5. The polyphase transient voltage suppressor set forth in claim 2 wherein said base member has a pair of parallel major surfaces, said plurality of members projecting from a first of the major surfaces of said base member.
 6. The polyphase transient voltage suppressor set forth in claim 2 wherein said plurality of projecting members are each of equal thickness dimension.
 7. The polyphase transient voltage suppressor set forth in claim 1 wherein said plurality of projecting members are of unequal thickness dimension.
 8. The polyphase transient voltage suppressor set forth in claim 6 wherein base member is of thickness dimension equal to 1/2 sin( pi /n) times the thickness dimension of one of said projecting members where n is the number of phases.
 9. The polyphase transient voltage suppressor set forth in claim 2 wherein said plurality of members project radially outward along the major surface of said base member.
 10. The polyphase transient voltage suppressor set forth in claim 9 wherein said plurality of projecting members have a common junction projecting perpendicularly outward from the center of the major surface of said base member.
 11. The polyphase transient voltage suppressor set forth in claim 10 wherein said base member is a right circular cylinder having a pair of parallel major surfaces, and the plurality of projecting members each having a curved side surface whiCh are continuous with the side surface of said base member and perpendicular to the major surface of the base member.
 12. The polyphase transient voltage suppressor set forth in claim 2 wherein said plurality of projecting members each having a side surface projecting from a side surface of said base member in a direction perpendicular thereto.
 13. The polyphase transient voltage suppressor set forth in claim 10 wherein said plurality of projecting members each have an end surface remote from said base member, the end surfaces of said plurality of projecting members each being parallel to the major surface of said base member.
 14. The polyphase transient voltage suppressor set forth in claim 13 wherein the end surfaces of said projecting members being coplanar to form a continuous flat surface parallel to the major surface of said base member.
 15. The polyphase transient voltage suppressor set forth in claim 14 wherein said plurality of projecting members each being of rectangular shape.
 16. The polyphase transient voltage suppressor set forth in clam 5 wherein said plurality of projecting members are three in number for forming a three phase transient voltage suppressor.
 17. The polyphase transient voltage suppressor set forth in claim 16 wherein said three projecting members being oriented at 120* angles around the center of the major surface of said base member whereby the end surfaces of said three projecting members form a Y-shape.
 18. The polyphase transient voltage suppressor set forth in claim 1 wherein the sintered metal oxide material has an alpha exponent in excess of
 10. 19. The polyphase transient voltage suppressor set forth in claim 1 wherein the sintered metal oxide material is comprised primarily of zinc oxide.
 20. The polyphase transient voltage suppressor set forth in claim 1 wherein said electrode means comprise an electrically conductive material formed on the major surfaces of said plurality of projecting members.
 21. The polyphase transient voltage suppressor set forth in claim wherein the electrically conductive material on adjoining major surfaces of adjacent projecting members are interconnected to form a like plurality of electrodes, and a like plurality of electrical conductors having first ends connected to the electrodes and second ends connected to the polyphase power lines to thereby provide polyphase line-to-line transient voltage suppression.
 22. The polyphase transient voltage suppressor set forth in claim 5 wherein said electrode means comprise an electrically conductive material formed on the major surfaces of said plurality of projecting members and on portions of said first major surface of said base member, and on a second of the pair of major surfaces of said base member, the electrically conductive material on adjoining major surfaces of adjacent projecting members and on adjoining portions of the first major surface of said base member being interconnected to form a like plurality of electrodes, and a like plurality of electrical conductors having first ends connected to the electrodes and second ends connected to the phase lines of the polyphase power line, and additional electrical conductor having a first end connected to the electrode on the second major surface of the base member and a second end connected to the power line neutral to thereby provide polyphase line-to-line and line-to-neutral transient voltage suppression.
 23. The polyphase transient voltage suppressor set forth in claim 21 wherein said plurality of projecting members are each of equal thickness dimension and are three in number for forming a three phase transient voltage suppressor, the electrically conductive material on adjoining major surfaces of adjacent projecting members being interconnected to form three electrodes, and three electrical conductor having first ends connected to the electrodes and second ends conneCted to a three phase power line or three phase power input or output of an apparatus to be protected to thereby provide three phase line-to-line transient voltage suppression.
 24. The polyphase transient voltage suppressor set forth in claim 22 wherein said plurality of projecting members are each of equal thickness dimension and are three in number for for phase three phase transient voltage suppressor, the electrically conductive material on adjoining portions of the first major surface of said base member being interconnected with the electrically conductive material on adjoining major surfaces of adjacent projecting members, and said plurality of electrical conductors are three in number, the additional conductor being a fourth conductor having a first end connected to the electrode formed on the second major surface of said base member and a second end connected to the three phase power line neutral to thereby provide three phase line-to-line and line-to-neutral transient voltage suppression.
 25. The polyphase transient voltage suppressor set forth in claim 24 wherein the electrically conductive material on adjoining major surfaces of at least one pair of adjacent projecting member are not being interconnected and also separate from the electrically conductive material on the adjoining portion of the first major surface of said base member, the electrical conductor associated therewith being connected to the electrically conductive material on the adjoining portion of the first major surface of said base member, whereby selected line-to-line transient voltage suppression is obtained.
 26. The polyphase transient voltage suppressor set forth in claim 25 and further comprising at least one semiconductor device mounted on said suppressor and having a first power electrode connected to the electrically conductive material on at least one of said one pair of adjacent projecting members and having a second power electrode connected to the electrically conductive material on the separated adjacent portion of the first major surface of said base member whereby the phase of the power line associated therewith is polarity sensitive to voltage transients on the two other phase conductors.
 27. The polyphase transient voltage suppressor set forth in claim 5 wherein said electrode means comprise an electrically conductive material formed on the major surfaces of said plurality of projecting members and base member, and further comprising cooling means associated with said plurality of projecting members and base member for obtaining higher power operation of the suppressor. 